[PATCH 7/7] SLIM: documentation
From: Kylene Jo Hall
Date: Wed Aug 23 2006 - 15:03:38 EST
Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxx>
Signed-off-by: Kylene Hall <kjhall@xxxxxxxxxx>
Documentation/slim.txt | 136 +++++++++++++++++++++++++++++++++++++++
1 files changed, 136 insertions(+)
--- linux-2.6.18/Documentation/slim.txt 1969-12-31 16:00:00.000000000 -0800
+++ linux-2.6.18-rc4/Documentation/slim.txt 2006-08-22 14:48:12.000000000 -0700
@@ -0,0 +1,136 @@
+Simple Linux Integrity Model (SLIM)
+SLIM is an LSM module which provides an enhanced low water-mark
+integrity and high water-mark secrecy mandatory access control
+model. It also is a consumer of the new integrity subsystem,
+using the integrity_verify_data(), integrity_verify_metadata(),
+and integrity_measure() calls to base mandatory access control
+decisions on the verified integrity status of the involved objects.
+SLIM is an extension of several prior models, including Biba,
+Lowmac, and Caernarvon, which provide excellent background.
+SLIM's specific model is:
+ All objects (files) are labeled with extended attributes to indicate:
+ Integrity Access Class (IAC)
+ (one of SYSTEM, USER, UNTRUSTED)
+ Secrecy Access Class (SAC)
+ (one of PUBLIC, USER, USER_SENSITIVE,
+ All processes inherit from their parents:
+ Integrity Read Access Class (IRAC)
+ Integrity Write/Execute Access Class (IWXAC)
+ Secrecy Write Access Class (SWAC)
+ Secrecy Read/Execute Access Class (SRXAC)
+ SLIM enforces the following Mandatory Access Control Rules:
+ IRAC(process) <= IAC(object)
+ SRXAC(process) >= SAC(object)
+ IWXAC(process) >= IAC(object)
+ SWAC(process) <= SAC(process)
+ IWXAC(process) <= IAC(object)
+ SRXAC(process) >= SAC(object)
+In the low water-mark model, rather than blocking attempted
+reads of lower integrity objects, the reading process is demoted
+to the integrity level of the object, so that the read is allowed.
+In a Linux client, this provides a much more usable environment,
+in which applications run more transparently, while being demoted
+as needed to protect the integrity of the system.
+When the process is demoted, it may have objects open for write
+of now higher integrity level, and these objects have to have their
+write access revoked. This revocation of write privilege must
+occur for normal and mmap'ed file writes. Similarly, when reading
+an object of higher secrecy, the process is promoted to the higher
+secrecy level, and write access to now lower secrecy objects is revoked.
+SLIM performs a generic revocation operation, including revoking
+mmap and shared memory access. Note that during demotion or promotion
+of a process, SLIM needs only revoke write access to files with higher
+integrity, or lower secrecy.
+SLIM inherently deals with dynamic task labels, which is a feature
+not currently available in selinux. While it might be possible to
+add support for this to selinux, it would not appear to be simple,
+and it is not clear if the added complexity would be desirable
+just to support this one model.
+Comments on the model:
+Some of the prior comments questioned the usefulness of the
+low water-mark model itself. Two major questions raised concerned
+a potential progression of the entire system to a fully demoted
+state, and the security issues surrounding the guard processes.
+In normal operation, the system seems to stabilize with a roughly
+equal mixture of SYSTEM, USER, and UNTRUSTED processes. Most
+applications seem to do a fixed set of operations in a fixed domain,
+and stabilize at their appropriate level. Some applications, like
+firefox and evolution, which inherently deal with untrusted data,
+immediately go to the UNTRUSTED level, which is where they belong.
+In a couple of cases, including cups and Notes, the applications
+did not handle their demotions well, as they occured well into their
+startup. For these applications, we simply force them to start up
+as UNTRUSTED, so demotion is not an issue. The one application
+that does tend to get demoted over time are shells, such as bash.
+These are not problems, as new ones can be created with the
+windowing system, or with su, as needed. To help with the associated
+user interface issue, the user space package README shows how to
+display the SLIM level in window titles, so it is always clear at
+what level the process is currently running.
+As for the issue of guard processes, SLIM defines three types of
+guard processes: Unlimited Guards, Limited Guards, and Untrusted
+Guards. Unlimited Guards are the most security sensitive, as they
+allow less trusted process to acquire a higher level of trust.
+On my current system there are two unlimited guards, passwd and
+userhelper. These two applications inherently have to be trusted
+this way regardless of the MAC model used. In SLIM, the policy
+clearly and simply labels them as having this level of trust.
+Limited Guards are programs which cannot give away higher
+trust, but which can keep their existing level despite reading
+less trusted data. On my system I have seven limited guards:
+yum, which is trusted to verify the signature on an (untrusted)
+downloaded RPM file, and to install it, login and sshd, which read
+untrusted user supplied login data, for authentication, dhclient
+which reads untrusted network data, and updates they system
+file /etc/resolv.conf, dbus-daemon, which accepts data from
+potentially untrusted processes, Xorg, which has to accept data
+from all Xwindow clients, regardless of level, and postfix which
+delivers untrusted mail. Again, these applications inherently
+must cross trust levels, and SLIM properly identifies them.
+As mentioned earlier, cupsd and notes are applications which are
+always run directly in untrusted mode, regardless of the level of
+the invoking process.
+The bottom line is that SLIM guard programs inherently do security
+sensitive things, and have to be trusted. There are only a small
+number of them, and they are clearly identified by their labels.
+Papers and slides on SLIM, along with source code for the needed
+userspace tools, and installation instructions are available at:
+[1 Biba]: K. J. Biba. âIntegrity Considerations for Secure Computer Systemsâ
+Technical Report ESD-TR-76-372, USAF Electronic Systems Division, Hanscom Air
+Force Base, Bedford, Massachusetts, April 1977.
+[2 Lomac]: T. Fraser, "LOMAC: Low Water-Mark Integrity Protection for COTS
+Environments," Proceedings of the 2000 IEEE Symposium on Security and
+Privacy, Oakland, California, USA, 2000.
+[3 Caernarvon]: P. Karger, V. Austel, and D. Toll. âUsing a Mandatory Secrecy
+and Integrity Policy on Smart Cards and Mobile Devicesâ EUROSMART Security
+Conference. 13-15 June 2000, Marseilles, France p. 134-148.
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